Optical information medium

ABSTRACT

In the multi-layer information medium having a plurality of information-storing layers according to the present invention, distance between the adjacent information-storing layers is reduced with no increase in cross talk between these layers. The optical information medium has at least two information-storing layers each storing recorded information and/or servo information, and at least one of the information-storing layers is recorded or read by the recording beam or the reading beam which has passed through other information-storing layer(s). The medium has a filter layer between the adjacent information-storing layers, and in the spectral absorption characteristics in the wavelength range of 300 to 1000 nm of this filter layer, a wavelength range exhibiting an absorption of 80% or higher and a wavelength range exhibiting an absorption of 20% or lower are present.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

This invention relates to a multi-layer information medium which has atleast two information-storing layers, the information-storing layerbeing typically a recording layer.

There is a growing need for an optical disk having a higher density anda higher capacity. DVD (Digital Versatile Disk) is already commerciallyavailable, and the DVD has a storage capacity of about 4.7 GB per singleside which is about seven times larger than the compact disk.Technologies enabling further increase in the amount of informationrecorded are under active development.

Technologies that have been used for increasing the recording capacityof an optical disk include use of a recording/reading beam having areduced wavelength, use of an objective lens having a higher NA(numerical aperture) in the optical system irradiating therecording/reading beam, increase in the number of recording layers, andmulti-value recording. Among these, three-dimensional recording byincreasing the number of recording layers enables remarkable increase inthe recording capacity at low cost compared to the use of shorterwavelength or use of a lens with a higher NA. The three dimensionalrecording medium is described, for example, in Japanese PatentApplication Kokai (JP-A) 198709/1997, and JP-A 255374/1996 discloses amedium wherein a rewritable information storage layer and a read onlyinformation storage layer are laminated.

In the reading of a multi-layer recording medium including a pluralityof recording layers by using an optical pickup which emits a readingbeam, the optical pickup receives the beam reflected from the recordinglayer on which the reading beam had focused, and in addition, the beamreflected from the recording layer(s) other than the recording layer towhich the reading beam had focused. This results in the signalinterference between the plurality of recording layers, and cross talkis induced. As a consequence, noise is introduced in the read outsignal. The influence of the beam reflected from the recording layerother than the target recording layer reduces inversely with the squareof the distance between the recording layers. Therefore, increase in thedistance between the adjacent recording layers is effective in reducingthe noise induced. For example, when the medium is used with an opticalpickup having the structure normally employed in DVD and otherconventional optical disks, the recording layers are disposed at amutual distance of at least 30 μm, and preferably at least 70 μm torealize the signal quality of practically acceptable level. Thiscorresponds the Examples of the JP-A 198709/1997 wherein a transparentresin layer of 100 μm thick is provided between the recording layers,and the JP-A 255374/1996 wherein two adjacent information storage layersare disposed at a distance of 30 μm or more.

However, when the distance between the adjacent recording layers isincreased to as large as 30 μm or more, limitation in the number ofrecording layers in the medium will be required to avoid excessiveincrease in the disk thickness, and the total storage capacity of thedisk will also be limited.

The transparent resin layer provided between the recording layers isalso associated with a difficulty. To be more specific, formation of atransparent resin layer with a consistent thickness is difficult inspite of various attempts in forming the transparent resin layer by spincoating, resin sheet disposition and the like when the transparent resinlayer formed is as thick as, for example, 30 μm or more, and inparticular, 70 μm or more. The thick resin layer also suffers fromincreased internal stress and the medium is subject to warping. As aconsequence, reliable provision with the optical disk of the requiredmechanical precision has been difficult.

In the case of the medium having a single recording layer formed on asubstrate, the shape of the grooves (guide grooves) formed in the resinsubstrate will be transferred to the recording layer. In contrast, inthe case of a medium wherein two or more recording layers are formed onthe substrate with an intervening relatively thick transparent layerbetween the recording layers, it is quite difficult to transfer theshape of the grooves formed in the substrate to all of the recordinglayers since the groove depth is about 100 nm at most for opticalreasons while the distance between the recording layers is at least 30μm as described above. As a consequence, formation of the grooves in thetransparent resin layer by photopolymerization (2P) process will berequired as described, for example, in the JP-A 198709/1997 and eminentincrease in the production cost is invited.

SUMMARY OF THE INVENTION

An object of the present invention is to reduce the distance betweenadjacent information-storing layers in a multi-layer information mediumhaving a plurality of data layers with no increase in the cross talkbetween the information-storing layers. Another object of the inventionis to provide such multi-layer information medium at a low cost.

Such objects are attained by the present invention as described in (1)to (5), below.

(1) An optical information medium having at least twoinformation-storing layers each storing recorded information and/orservo information, wherein at least one of the information-storinglayers is recorded or read by the recording beam or the reading beamwhich has passed through other information-storing layer(s), and whereinthe medium has a filter layer between the adjacent information-storinglayers, wherein

in the spectral absorption characteristics in the wavelength range of300 to 1000 nm of this filter layer, a wavelength range exhibiting anabsorption of 80% or higher and a wavelength range exhibiting anabsorption of 20% or lower are present.

(2) An optical information medium having at least twoinformation-storing layers each storing recorded information and/orservo information, wherein at least one of the information-storinglayers is recorded or read by the recording beam or the reading beamwhich has passed through other information-storing layer(s), wherein themedium has a filter layer between the adjacent information-storinglayers, and wherein the medium is used in a system wherein two or morerecording/reading beams each having different wavelength are used,wherein

the filter layer exhibits a relatively high absorption for thereading/recording beam used for its closest information-storing layer onthe side of the light incidence and a relatively low absorption for thereading/recording beam used for its closest information-storing layer onthe side of the light exit.

(3) The optical information medium according to the above (2) whereinsaid filters exhibits an absorption for the reading/recording beam usedfor its closest information-storing layer on the side of the lightincidence of 80% or more and an absorption for the reading/recordingbeam used for its closest information-storing layer on the side of thelight exit of 20% or lower.

(4) The optical information medium according to any one of the above (1)to (3) wherein at least one of said filter layers is a layer formed byUV curing a composition containing a UV-curable composition and aphotopolymerization initiator.

(5) The optical information medium according to any one of the above (1)to (4) wherein at least one of said filter layers is a layer containinga dye.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of the optical informationmedium according to an embodiment of the present invention.

FIG. 2 is a partial cross-sectional view of the optical informationmedium according to an embodiment of the present invention.

FIG. 3 is a partial cross-sectional view of the optical informationmedium according to another embodiment of the present invention.

FIG. 4 is a view showing an embodiment of the optical pickup used forrecording and reading of the optical information medium according to thepresent invention.

FIG. 5 is a view showing another embodiment of the optical pickup usedfor recording and reading of the optical information medium according tothe present invention.

FIG. 6 is a cross-sectional view showing a step in the formation of thetransparent layer.

FIG. 7 is a cross-sectional view showing a step in the formation of thetransparent layer.

FIG. 8 is a cross-sectional view showing a step in the formation of thetransparent layer.

FIG. 9 is a cross-sectional view showing a step in the formation of thetransparent layer.

FIG. 10 is a cross-sectional view showing a step in the formation of thetransparent layer.

FIG. 11 is a cross-sectional view showing a step in the formation of thetransparent layer.

FIG. 12 is a cross-sectional view showing the medium near the innerperiphery of the substrate where the transparent layers and the datalayers have been formed.

FIG. 13 is a cross-sectional view showing a procedure for forming thesecond transparent layer.

FIGS. 14A to 14D are cross-sectional views showing various embodimentsof the plug means.

The information medium to which the present invention is applied has astructure comprising at least two information-storing layers. The term“information-storing layer” used herein includes both the data layer andthe servo layer. A “data layer” is the layer wherein record marks andpits carrying the recorded information are present, and “a servo layer”is the layer formed with a tracking servo pattern comprising projectionsand depressions such as grooves and pits. When the servo layer is notprovided independently from the data layer, tracking servo pattern maybe formed on the data layer.

In the present invention, the beam used in the reading of the data layerand the recording of the data layer is designated the “data beam” andthe beam used in the reading of the servo layer is designated the “servobeam”. The “recording/reading beam” of the present invention is aconcept including the data beam and the servo beam.

The “multi-layer information medium” of the present invention is amedium comprising a plurality of information-storing layers, and whichincludes an information-storing layer whose recording and/or reading isaccomplished by the recording/reading beam which has passed throughother information-storing layer(s).

The “optical information medium” of the present invention includes bothan optical recording medium and a read only medium. In the case of anoptical recording medium, the data layer includes a recording layer. Inthe case of a read only medium, data-storing pits or record marks arepreliminarily formed in the data layer.

FIG. 1 shows an embodiment of the multi-layer medium of the presentinvention in its cross section. The medium of FIG. 1 comprises asubstrate 2 formed with tracking grooves, two data layers DL-1 and DL-2disposed on the substrate 2, a filter layer FL between the two datalayers, and a transparent layer TL on the data layer DL-2. Thetransparent layer TL functions as a protective layer. In this medium,the data layers DL-1 and DL-2 are respectively read by introducing tworeading beams each having different wavelength from the bottom side ofthe medium in the drawing and detecting the reflected beam by theoptical pickup. When the medium is an optical recording medium, therecording beam and the reading beam are generally emitted from the sameoptical pickup, and the recording and the reading beams generally havethe same wavelength.

The filter layer FL of the medium of the present invention shown in FIG.1 exhibits an absorption for the data beam used for reading the lowerdata layer DL-1 which is higher than the absorption for the data beamused for reading the upper data layer DL-2. As a consequence, in thereading of the data layer DL-1, intensity of the reading beam reachingthe data layer DL-2 is reduced and the influence of the beam reflectedfrom the data layer DL-2 would be suppressed. In contrast, in thereading of the data layer DL-2, the data beam is not so much absorbed bythe filter layer FL and the reading is not obstructed. This enablesprovision of the data layer DL-1 and data layer DL-2 in close proximityto each other with reduced cross talk being induced between these datalayers. In contrast, if a transparent layer highly transparent to therecording/reading beam were formed instead of the filter layer FL, theoptical pickup will pick the beam reflected from the upper data layerDL-2 while the lower data layer DL-1 is read by focusing at the lowerdata layer DL-1, and this will result in a reading noise unless thethickness of the transparent layer were fully increased.

As a matter of fact, in the reading of the upper data layer DL-2, theinfluence of the beam reflected from the lower data layer DL-1 isinescapable. The cross talk influence, however, will be reduced when therecording density is low, and the medium of the embodiment of FIG. 1 ispreferably designed such that the recording density of the DL-2 is lowerthan that of the DL-1. In such a case, a beam with a longer wavelengthis generally used as the data beam for recording/reading of the DL-2compared to the data beam used for the recording/reading of the DL-1.

FIG. 2 shows the medium according to another embodiment of the presentinvention. The medium shown in FIG. 2 comprises a substrate 2, one datalayer DL on the substrate 2, a filter layer FL on the data layer DL, anda servo substrate 20 on the filter layer FL. The servo substrate 20 isformed with a tracking servo pattern comprising grooves and/or pits, andon the surface on the side of the recording/reading beam incidence ofthis servo substrate 20 is formed a reflective layer which functions asthe servo layer SL.

In the reading of the medium shown in FIG. 2, a beam having a wavelengthdifferent from the data beam used in reading the data layer DL is usedfor the servo beam used in reading the servo layer SL. The filter layerFL of this medium has an absorption for the data beam higher than theabsorption for the servo beam, and in the reading of the data layer DL,there is less likeliness for contamination of the reading noise inducedby the data beam reflected from the servo layer SL.

The reading of the servo information such as tracking servo informationis less likely to be influenced by the noise compared to the reading ofthe data layer, and in the embodiment shown in FIG. 2, reading of thedata layer recorded at a high recording density at low noise is realizedsimultaneously with high precision servo. In addition, since the servolayer is independently formed in the embodiment of FIG. 2, formation ofthe data layer as a smooth layer is enabled. As a consequence, thereflectivity of the data layer DL is increased, and no interference isinduced by the steps of the tracking servo pattern. Generation of thenoise due to irregularity such as deformation of the tracking servopattern such as winding of the groove is also avoided. In the embodimentof FIG. 2, a beam with a shorter wavelength is generally used for thedata beam compared to the servo beam.

FIG. 4 shows an embodiment of the optical pickup which can be used inthe recording and reading of the multi-layer information medium of thepresent invention together with the medium having the structure of FIG.2.

In this optical pickup, the data beam is emitted from a laser diode LD1.The data beam then goes through a lens L1 to become collimated, andafter going through a polarizing beam splitter PBS 1, the beam passesthrough quarter-wave plate QWP 1 and dichroic mirror DCM which istransparent to the data beam, and the beam enters objective lens L4 tobe focused at the data layer DL of the multi-layer information medium.The data beam reflected by the data layer DL goes back along the samepathway as the incidence into the medium, and the beam is then reflectedby the polarizing beam splitter PBS 1 to be focused by a lens L5 to aphotodetector PD 1. The focus servo to the data layer DL, or the focusservo and detection of the signal that has been read is therebyaccomplished.

In the medium of FIG. 4, a filter layer FL is present between the datalayer DL and the servo layer SL, and the data beam returning to theoptical pickup after being reflected at the servo layer SL will havegone through the filter layer FL and back to become significantlyattenuated. Generation of the noise in the reading of the data layer DLdue to the reflection at the servo layer is thereby suppressed to aconsiderable degree.

In the meanwhile, the servo beam is emitted from a laser diode LD2. Thebeam is then reflected by a polarizing beam splitter PBS 2, and goesthrough a lens L6 and a quarter-wave plate QWP 2 to be reflected by adichroic mirror DCM. The beam then enters the objective lens L4 tobecome focused on the servo layer SL. The servo beam is then reflectedby the servo layer SL to go back along the same pathway as its incidenceinto the medium, and the beam passes through the polarizing beamsplitter PBS 2 to be focused on a photodetector PD 2. The tracking servoand the focus servo to the servo layer are thereby accomplished.

Use of the optical pickup of such constitution, namely, the opticalpickup equipped with the dichroic mirror DCM which has spectralcharacteristics of reflecting the servo beam while allowing the databeam to pass therethrough is advantageous when the data layer and theservo layer are separately provided and the data beam and the servo beamare simultaneously irradiated for the reading. In this way, introductionof the reflected servo beam into the photodetector PD1 provided fordetection of the data beam as well as introduction of the reflected databeam into the photodetector PD2 provided for detection of the servo beamcan be avoided.

The dichroic mirror DCM, however, is not capable of fully passing thedata beam therethrough, and the data beam is partly reflected by thedichroic mirror DCM. If a transparent layer were provided instead of thefilter layer FL shown in FIG. 4, the data beam would partly reach thephotodetector PD 2 provided for the servo purpose to adversely affectthe tracking servo. When the data beam has a high intensity as in thecase of the data beam used in the recording, such adverse effect isserious. In contrast, when a filter layer FL is provided between thedata layer DL and the servo layer SL, the data beam is considerablyattenuated on the way and back through the filter layer FL, and theadverse effects on the tracking servo caused by the data beam is greatlysuppressed.

FIG. 3 shows another embodiment of the multi-layer information mediumaccording to the present invention. The medium of FIG. 3 comprises asubstrate 2, five transparent layers TL-1 to TL-5 on the substrate 2,and four data layers DL-1 to DL-4 between the adjacent transparentlayers. On the transparent layer TL-5 is formed a filter layer FL, aservo layer SL, and a servo substrate 20 in this order. The servosubstrate 20 is formed with a tracking servo pattern comprising groovesand/or pits, and this pattern is transferred to the servo layer SL.

The medium of FIG. 3 has a constitution similar to that of FIG. 2 exceptfor the larger number of data layers. When two or more data layer areformed, and in particular, when three or more data layers are formed,formation of the tracking servo pattern of high precision in each of thedata layer at a low cost is difficult, and the structure wherein thedata layers and the servo layer are independently formed is effective.

In the medium of FIG. 3, the filter layer FL is provided between thedata layer DL-4 and the servo layer SL, and no filter layer is providedbetween adjacent data layers. Therefore, provision of the data layers ata close proximity invites increase in the cross talk. When the crosstalk is to be reduced in the medium of such structure, use of an opticalpickup having a confocal optical system which utilizes the principle ofa confocal microscope is desirable. An optical pickup having a confocaloptical system has a very high resolution in the thickness direction ofthe medium, and the cross talk between the data layers can be greatlyreduced by the use of such optical pickup. Confocal optical systemswhich may be used in the reading of a multi-layer information medium aredescribed, for example, in JP-A 222856/1998 and SOM '94 technical digest(1994) 19.

An embodiment of the optical pickup which is equipped with a confocaloptical system and which can be used in the recording and reading of amulti-layer information medium is shown in FIG. 5 together with themedium. The medium shown in FIG. 5 has a structure comprising asubstrate 2, and a data layer DL-1, a transparent layer TL, a data layerDL-2, a filter layer FL, a servo layer SL, and a servo substrate 20disposed on the substrate 2 in this order.

This optical pickup has a structure similar to the optical pickup shownin FIG. 4 except that a lens L2, a pin-hole plate PHP, and a lens L3have been incorporated between the polarizing beam splitter PBS 1 andthe quarter-wave plate QWP1 in the light path of the data beam.

In this optical pickup, the data beam which has passed through thepolarizing beam splitter PBS 1 is focused by the lens L2. A pin-holeplate PHP formed with a pin hole is arranged at the focal point, and thedata beam which has passed through the pin hole is collimated by thelens L3 and after passing through the pathway similar to that of theoptical pickup shown in FIG. 4, the beam is focused at the data layerDL-1 on the lower side of the multi-layer information medium. The databeam reflected by the data layer DL-1 goes back along the same pathwayas the incidence into the medium. The data beam also reaches the datalayer DL-2 after passing through the data layer DL-1 the data of whichis to be read, and the beam is also reflected from the data layer DL-2back to the optical pickup. This data beam, however, is out of focus atthe data layer DL-2, and the beam reflected from the data layer DL-2 isnot focused to the pinhole position of the pinhole plate PHP. The beamwhich failed to be unfocused at the pinhole is substantially blocked bythe pinhole plate PHP. The cross talk between the data layers is therebysuppressed by the optical pickup equipped with the confocal opticalsystem.

Next, constitution of various parts of the optical recording medium ofthe present invention is described in detail.

Filter Layer

The filter layer shown in FIGS. 1 to 3 is a layer which exhibits theabsorption for one of the two recording/reading beams (two data beams,or one data beam and one servo beam) higher than the absorption for theother beam. To be more specific, the absorption of the filter layer forone recording/reading beam is preferably 80% or higher and morepreferably 90% or higher, and the advantage of the present invention isnot fully realized when this absorption is too low. On the other hand,the absorption of the other recording/reading beam is preferably 20% orlower and more preferably 10% or lower, and when this absorption is toohigh, reading of the information-storing layer by the recording/readingbeam which has gone through the filter layer will be difficult,rendering the recording of the medium difficult in the case of arecording medium.

The material used for the filter layer is not particularly limited, andan adequate material may be selected from the materials exhibiting thedesired spectral absorption characteristics, for example, from the dyescomprising an organic material or an inorganic material. Use of anorganic dye is preferable, and use of an organic dye further comprisinga resin is more preferable. Exemplary preferable resins include resinscurable with UV or other active energy ray. Formation of the filterlayer is facilitated by such admixture of the resin component comparedto the use of the dye alone. For example, a uniform, relatively thickfilter layer can be formed in a short period when a mixture of aUV-curable composition an a dye is spin coated and UV cured.

The dye used for the filter layer is not particularly limited, and anadequate dye may be selected from the dyes exhibiting the spectralabsorption characteristics required for a filter layer, for example,from cyanine, phthalocyanine, and azo organic dyes. The dye may bemodified as desired, for example, by introducing a substituent in theside chain of the dye in consideration of the compatibility with theresin. The filter layer may also comprise two or more dye layers eachhaving different spectral absorption characteristics for facilitatingthe control of the spectral absorption characteristics.

When the filter layer contains a dye and a resin, the dye is not limitedfor its content, and the content may be determined depending on the typeof the resin employed and to satisfy the required spectral absorptioncharacteristics. The content is typically 1 to 10 mass %. An excessivelylow dye content is undesirable since increase in the thickness of thefilter layer is required. On the other hand, excessively large contentwill result in the shortening of the pot life.

When the wavelength of the beam to be absorbed is relatively short, andto be more specific, when steep absorption is to be realized in thewavelength region of up to 450 nm, the filter layer may be constitutedfrom a UV-curable resin layer free from the dye. The UV-curable resinlayer may be formed by coating a composition containing a UV-curablecomposition and a photoinitiator, and UV curing the coated film. Thephotoinitiator exhibits high absorption near the wavelength of the lightbeam used for the curing, and the thus cured film also exhibits highabsorption near such wavelength. This is believed to be due to thecondition that the photoinitiator is not completely decomposed in thecourse of curing and a part of the photoinitiator remains in intact ormodified state after the curing. As a consequence, such layer can beused as a filter layer which exhibits selectively high absorption at theshort wavelength region.

The photoinitiator used in the filter layer is not particularly limited,and an adequate photoinitiator may be selected from conventionalphotoinitiators such as benzoates, benzophenone derivatives, benzoinderivatives, thioxanthone derivatives, acetophenone derivatives,propiophenone derivatives, and benzyls depending on the wavelength ofbeam to be absorbed.

The thickness of the filter layer may be adequately determined tosatisfy the required spectral absorption characteristics. However, thefilter layer containing a resin wherein a dye or a photoinitiator isused for the absorption material is preferably formed to a thickness inthe range of 1 to 30 μm. When the filter layer is too thin, sufficientabsorption characteristics is less likely to be obtained. When thefilter layer is too thick, number of the data layers included in themedium will be limited in view of the total thickness of the medium, andthis is not preferable.

When the wavelength of the beam to be absorbed is relatively short, forexample, up to 450 nm, a metal layer containing at least one metal(including semimetal) element may be used for the filter layer. Somemetals including gold exhibit steep high absorption in the shortwavelength region. In view of such situation, the type of the metalincluded and the thickness of the filter layer may be selected so thatsufficient absorption and sufficient transmittance are reliably achievedat the target wavelength regions of absorption and transmittance,respectively. Examples of the metals which may be preferably used in thefilter layer include Au, Pt, Cu and the like. The filter layer may alsocomprise two or more different metal layers each having differentspectral absorption characteristics.

The thickness of the metal layer used as the filter layer may vary bythe type of the metal used. However, the thickness of such layer ispreferably in the range of 10 to 200 nm, and more preferably 20 to 100nm. When the metal layer is too thin, the layer will fail to exhibitsufficient absorption at the target absorption wavelength region whileexcessively thick metal layer results in an insufficient transmittanceat the target wavelength region.

The filter layer may also comprise an interference filter. Exemplaryinterference filters which may be used include a dielectric multi-layerfilm and a dielectric layer sandwiched between two metal thin filmscomprising Ag or the like.

In the embodiment of FIG. 3, the filter layer is provided only betweenthe data layer and the servo layer, namely, only at one of the locationsbetween the adjacent information-storing layers. The filter layer,however, may be also formed at other locations between adjacentinformation-storing layers as desired. To be more specific, two or morefilter layers may be formed with three or more beams each havingdifferent wavelength being used for the recording or reading beam. Forexample, data layers DL-1, DL-2, and DL-3 may be formed in this orderfrom the side of the light incidence with filter layers FL-1 and FL-2respectively formed between the DL-1 and the DL-2 and between the DL-2and the DL-3, and the DL-1, the DL-2 and the DL-3 may be read with thebeams having a wavelength of 400 nm, 600 nm and 800 nm, respectively. Inthis case, the filter layer FL-1 is preferably the one exhibiting highabsorption to the beam with a wavelength at around 400 nm and lowabsorption to the beams at around 600 nm and 800 nm. The filter layerFL-2 is not limited for its absorption for the beam with the wavelengthat around 400 nm while it should exhibit high absorption at around 600nm and low absorption at around 800 nm.

To be more specific, in the case of a medium wherein the number of thefilter layers is “n” and which is to be used in a system wherein “n+1”recording/reading beams each having different wavelengths are used, thefilter layer should exhibit a relatively high absorption for thereading/recording beam used for its closest information-storing layer onthe side of the light incidence and a relatively low absorption for thereading/recording beam used for its closest information-storing layer onthe side of the light exit. The “relatively high absorption” used hereindesignates an absorption of preferably 80% or higher, and morepreferably 90% or higher, and the “relatively low absorption” usedherein designates an absorption of preferably 20% or lower, and morepreferably 10% or lower.

When two or more filter layers are formed, all of the filter layers maynot necessarily comprise the same type of optical absorbing material.The filter layers, for example, may comprise a combination of a metallayer or an interference filter and a dye-containing filter layer.

In the medium of FIG. 3, the reflective layer (the servo layer SL) onthe surface of the servo substrate 20 may be used as the filter layerinstead of forming a filter layer between the data layer and the servolayer. In the case when the present invention is used in a read onlymedium, the transparent layer or the filter layer may be formed withpits, and a translucent reflective layer may be formed on the surfaceformed with the pits by sputtering or the like to thereby use the thusformed reflective layer also as the data layer. In this case, thereflective layer formed from a metal or a semi-metal may be used as thefilter layer. In such case, the filter layer which also serves as theinformation-storing layer may preferably exhibit a relatively highreflectivity for the recording/reading beam used for the filter layeritself and a relatively low reflectivity for the recording/reading beamused for its closest information-storing layer on the side of the lightincidence. If an information-storing layer is also present on the sideof the light exit, the filter layer may preferably exhibit a relativelyhigh transmittance for the recording/reading beam used for suchinformation-storing layer.

The recording/reading beams having different wavelengths from each otherare not particularly limited for their wavelengths. However, differencein the wavelength between these recording/reading beams is preferably inthe range of 50 to 700 nm, and more preferably 100 to 400 nm. When thewavelength difference is too small, the filter layer will be required tohave steep spectral absorption characteristics and selection of thematerial used for the filter layer will be difficult. When thewavelength difference is too large, difficulty is encountered inincreasing the recording density of the entire medium or in obtainingsufficient servo accuracy.

The wavelength region wherein these recording/reading beams are presentis preferably the wavelength region of 300 to 1000 nm, and morepreferably 400 to 800 nm. A semiconductor laser oscillating a laser beamhaving a wavelength shorter than such range is difficult to obtain whileuse of a laser beam having a wavelength longer than such range isassociated with difficulty in high density recording as well asdifficulty in the reading of the information recorded at a high density.

Transparent Layer

The transparent layer in FIG. 3 preferably comprises a material whichexhibits high transmittance to the recording/reading beam. The materialused for the transparent layer is not limited. The transparent layer,however, is preferably formed from a resin since the transparent layershould be deposited to a considerable thickness. The process used forthe formation of the transparent layer is not limited. In view of theease of forming a uniform, transparent layer in short time, thetransparent layer is preferably formed from a resin, and in particular,from a UV-curable resin or other active energy beam-curable resin. Itshould be noted that the transparent layer may be also formed from aresin sheet.

When the transparent layer is provided in contact with the substrate 2,it should be noted that the difference between the refractive index ofthe transparent layer and the refractive index of the substrate is up to0.1 at the wavelength of the recording/reading beam in order to suppressthe reflection at the boundary between the transparent layer and thesubstrate.

The transparent layer formed from a UV-curable resin will exhibit arelatively steep absorption in the short wavelength region due to theinfluence of the photoinitiator as described above in the section of the“Filter layer”. In order to reliably impart the transparent layer with asufficient transparency to the recording/reading beam in the shortwavelength region, an adequate type of photoinitiator should be selecteddepending on the wavelength of the recording/reading beam used.

The transparent layer is not particularly limited for its thickness, andthe thickness may be adequately determined so that the cross talkbetween the data layers is within acceptable limits. Typically, thetransparent layer has a thickness of at least 30 μm when an opticalpickup of conventional type is used. An excessively thick transparentlayer is likely to result in an unduly increased inconsistency in thethickness as well as increased internal stress, and such a thicktransparent layer is also likely to invite increase in the totalthickness of the medium. Accordingly, the transparent layer preferablyhas a thickness of up to 100 μm.

On the other hand, when a confocal optical system is adopted, thethickness of the transparent layer is determined depending on theresolution of the confocal optical system in the depth direction so thatthe cross talk between the data layers is within acceptable limits. Tobe more specific, the preferable thickness of the transparent layer is 5μm or more when the data beam has a wavelength of about 300 to about1000 nm although such thickness may vary with the wavelength of the databeam and the constitution of the confocal optical system. Use of aconfocal optical system enables provision of a transparent layer with areduced thickness of less than 30 μm, and no problem is induced evenwhen the thickness is reduced to 20 μm or less.

When the medium has a disk shape and the transparent layer comprises aresin, the transparent layer is preferably formed by spin coating sincethe spin coating is a process which is capable of forming a relativelyuniform transparent layer. In the transparent layer formed by spincoating, however, the radially outer portion of the resin layer becomesthick in relation to the radially inner portion of the layer, and thetransparent layer would suffer from an increased thickness inconsistencyin radial direction. The number of the transparent layers increases withthe number of the data layers, and such thickness inconsistency isaccumulated with the increase in the number of the data layers. As aconsequence, even if the data beam entered the substrate 2 in the outerperipheral region of the disk at normal direction, the data beamreflected at the surface of the data layer will not be normal to thesubstrate 2, and the quantity of the light returning to the opticalpickup will be reduced. The medium will then exhibit different readingoutputs in the inner peripheral region and in the outer peripheralregion.

In the case of an optical pickup equipped with a confocal opticalsystem, a pinhole is provided in the optical system and the reading isaccomplished by using the beam that had passed thorough this pinhole.Accordingly, when an optical pickup equipped with a confocal opticalsystem is used, the range of the focus servo will be narrower, andtherefore, a higher thickness consistency is required for thetransparent layer.

In view of such situation, difference between the maximum thickness andminimum thickness of the transparent layer between recordedinformation-storing areas (area where the recording tracks are present)of two adjacent data layers or between the recorded information-storingarea of the data layer and the servo information-storing areas of theservo layer is preferably up to 3 μm, and more preferably up to 2 μm.When the thickness inconsistency of the transparent layer is reduced tosuch range, the fluctuation in the reading output can be criticallyreduced. Although it may be preferable that the difference between themaximum and minimum thickness of the transparent layer is reduced to thelowest possible value, reduction of such difference to zero is difficultas long as the transparent layer is formed by spin coating, and thefluctuation in the reading output is sufficiently reduced when thethickness difference is within the above-specified range. Therefore, thethickness difference need not be reduced to less than 1 μm. In thedisk-shaped medium, the recorded information-storing area is typicallyan annular area having a width of about 20 to about 50 mm.

It should be also noted that the resin layers other than the transparentlayer, for example, the filter layer comprising a resin or a resin and adye, a protective layer which is often provided on the surface of themedium, an adhesive layer, and the like may be formed by spin coating.These resin layers are also required to have reduced thicknessinconsistency as in the case of the transparent layer.

In order to reduce the thickness inconsistency of the transparent layer,the filter layer, and other resin layers, the step of the spin coatingis preferably accomplished by using the apparatus as described below.

The step of the spin coating is explained in the following by referringto an example wherein the transparent layer TL-1 is formed in the mediumof FIG. 3. In this method, a substrate 2 formed with a center hole 101is placed on a turntable 200 as shown in FIGS. 6 and 7. It should benoted that, when a transparent layer other than the TL-1 is formed, thesubstrate 2 has an information-storing layer or an information-storinglayer and a resin layer already formed thereon. The substrate 2 issecured to the turntable 200 with its center hole 101 fitted on anannular ridge 201 of the turntable 200. FIGS. 6 and 7 arecross-sectional views showing only the end faces appearing at thesection, and the views in depth direction are omitted. This also appliesto the following cross-sectional views.

Next, the center hole 101 is covered with a plug means 300. The plugmeans 300 has a disk member 301 for covering the center hole 101, asupport shaft 302 integrally formed with the disk member 301 at itscenter, and a projection 303 integrally formed with the disk member 301on the side facing the center hole 101. By fitting the projection 303inside the ridge 201, the plug means 300 is secured to the turntable 200and the substrate 2 can be positioned in relation to the plug means 300.The method used in securing the substrate 2 and the plug means 300 tothe turntable 200 is not particularly limited, and in an exemplarymethod, the substrate 2 is first fitted with the plug means 300, and theplug means 300 is then fitted in the turntable 200.

Next, a coating solution 500 comprising a resin or a resin solution ispoured as shown in FIG. 8 from a nozzle 400 which is a discharging meansto deliver the coating solution 500 to the outer periphery of thesupport shaft 302. The turntable 200 is simultaneously rotated at arelatively low speed, and preferably at a rotation speed of 20 to 100rpm so that the coating solution is uniformly distributed over the diskmember 301.

Next, the turntable 200 is rotated at a relatively high speed to spreadthe coating solution 500 on the substrate 2 as shown in FIG. 9 and tothereby form the transparent layer TL-1 on the substrate 2.

The conditions used for the spreading of the coating solution are notparticularly limited. It is known that the thickness of the coated filmtheoretically increases in relation to the square root of the coatingsolution viscosity if the spin coating is conducted such that theconditions other than the coating solution viscosity are identical. Onthe other hand, the coated film becomes thin with the increase in therotation speed and the rotation time. Therefore, the rotation speed andthe rotation time used in the spin coating may be adequately determineddepending on the thickness of the transparent layer TL-1 to be formedand the coating solution viscosity.

Next, the plug means 300 is separated from the substrate 2 as shown inFIG. 10. As the outer periphery of the disk member 301 leaves thesubstrate 2, the inner periphery of the transparent layer TL-1 islifted, and an annular raised rim 600 is formed as shown in FIG. 10. Theannular raised rim 600 is the portion wherein the resin constituting thetransparent layer TL-1 is continuously uplifted.

When the coating solution used contains a UV-curable resin, thetransparent layer TL-1 is cured by UV irradiation as shown in FIG. 11.Although the UV irradiation is conducted in FIG. 11 on the turntable200, the curing may also be conducted on a curing stage separatelyprovided from the turntable. Also, the separation of the plug means maybe accomplished while the substrate is rotated.

The annular raised rim 600 formed by such method has a cross section ofsmooth curve (arc) as shown in the drawings. On the contrary, if theplug means 300 is separated after the curing of the transparent layerTL-1, a continuous annular raised rim will not be formed, and even ifsome projections were formed at the corresponding position, theprojections formed will be burrs and not an annular raised rimcontinuously extending in peripheral direction. Such procedure alsoinvolves the problem that debris of the cured resin are likely toscatter over the substrate 2.

Height of the annular raised rim 600, namely, height of the top of theannular raised rim measured from the lowest part in the surface of theresin layer near the annular raised rim is typically in the range of 1to 100 μm. Width of the annular raised rim 600, namely, distance betweenthe lowest part in the surface of the transparent layer near the annularraised rim to the inner periphery of the transparent layer is typicallyin the range of 0.5 to 3 mm. It should be noted that the height and thewidth of the annular raised rim increases with the increase in thethickness of the resin layer.

After completing the formation of the first transparent layer TL-1,first data layer DL-1 is formed by means of sputtering or the like. Thedata layer is formed such that the inner periphery of the data layer islocated at a position radially outside the inner periphery of theunderlying transparent layer.

Next, a second transparent layer TL-2 is formed by using the plug means300 again. This time, the annular raised rim 600 is already present atthe inner periphery of the first transparent layer TL-1. If the plugmeans 300 used is the same as the one used in the formation of the TL-1,spreading of the resin will be inhibited by the annular raised rim 600and formation of the TL-2 is likely to be interfered. In addition, theannular raised rim formed in TL-2 will be formed over the annular raisedrim of the TL-1 to result in great deviation of the resin layerthickness near the disk inner periphery from the designed value, and thedistance between the data layers will be increased in the region nearthe inner periphery.

In order to solve such problem, the resin layers are formed in thepresent invention such that the annular raised rims of the resin layersare located in mutually displaced relationship. FIG. 12 is across-sectional view of the substrate 2 near its inner periphery whereinthe transparent layers TL-1 to TL-4 and the data layers DL-1 to DL-4have been alternately disposed. In FIG. 12, the transparent layer remotefrom the substrate 2 has a larger inner diameter, and as a consequence,the inner periphery of the transparent layer laminate after theformation of the transparent layers has a step-like configuration, andthe annular raised rims 600 are left exposed on the surface of the thusformed steps. As described above, the problem as described above can besolved by disposing the transparent layers in the form of steps so thatthe subsequently formed transparent layer does not cover the annularraised rim of the preceding transparent layer.

In order to form a transparent layer laminate having the inner peripheryof step-like configuration, the second transparent layer TL-2 may beformed by using a plug means 300 as shown in FIG. 13. The procedureshown in FIG. 13 is substantially the same as the procedure shown inFIG. 6 except for the use of the substrate 2 which is already formedwith the transparent layer TL-1. The plug means 300 used, however, isdifferent. The disk member 301 of this plug means 300 has a diameterlarger than the one shown in FIG. 6 in order to form a transparent layerhaving an inner diameter larger than the transparent layer TL-1. Thedisk member 301 is also bored in its lower surface so that the memberwill be in contact with the flat area of the transparent layer TL-1 bybridging the annular raised rim 600. The third and the followingtransparent layers may be formed by using a plug means having a diskmember of similar configuration but of the size which can cover theannular raised rim of the preceding transparent layer.

The plug means used in the procedure as described above is not limitedfor its constitution as long as its includes a disk member for blockingthe center hole of the disk substrate. Spin coating methods using a plugmeans for blocking the center hole of the disk substrate are described,for example, in JP-A 320850/1998, JP-A 249264/1998, JP-A 289489/1998,JP-A 195250/1999, and JP-A 195251/1999. These documents disclose methodswherein the center hole of the disk is blocked with a plug means such asa plate-shaped substrate, a disk member, a plug member, or a cap, andthe resin is supplied to a location near the center of such plug means,namely, near the rotation center to thereby reduce the thicknessunevenness in radial direction of the resin layer. However, thesedocuments refer neither to the multi-layer information medium nor to theformation of annular raised rim at the inner periphery of the resinlayer in the spin coating. In addition, the plug means described inthese documents suffer from the problems as described below.

Of these documents, JP-A 320850/1998, JP-A 249264/1998, and JP-A195250/1999 do not disclose removal of the plug means, namely, theplate-shaped member or the cap after the spin coating, and employment ofsuch methods in an industrial production is difficult. These documentsare also silent about the curing of the resin layer after the separationof the plug means from the disk substrate.

JP-A 289489/1998 discloses curing of the resin layer while rotating thedisk substrate after the spin coating and removal of the plate-shapedmember, namely, the plug means by knocking or electromagnetic suction.The removal of the plug means by knocking or electromagnetic suction,however, applies a considerable acceleration to the plug means, and thisis likely to cause turbulence in the resin coating film.

JP-A 195251/1999 discloses a plug means comprising a spherical cap and asupport member integrally formed with the spherical cap at its center.JP-A 195251/1999 teaches that this support member facilitates theattachment/detachment and the positioning of the plug means. The supportmember of JP-A 195251/1999 comprises either a hollow cylinder providedwith at least one opening or a plurality of rods. The resin layer isformed on the disk substrate by introducing the resin into the interiorof the hollow cylinder or into the area surrounded by the plurality ofrods, and rotating the disk substrate together with the plug means.These plug means certainly facilitate removal of the plug means. JP-A195251/1999 discloses curing of the resin layer in stationary stateafter separating the plug means from the disk substrate.

In JP-A 195251/1999, the spin coating is accomplished by allowing theresin to flow from the openings provided on the hollow cylinder of theplug means or from the intervals between adjacent rods of the plugmeans. Therefore, the resin is once stopped at the wall (i.e. area ofthe wall other than the opening) or rods of the support member, and thestopped resin may sweep onto the disk substrate at an unexpected timingto result in an uneven coating. In addition, these plug means aredifficult to clean since the part that becomes in contact with the resinis quite complicated in shape and a large area is brought in contactwith the resin. The resin remaining on the plug means may become a causeof an uneven coating. Furthermore, as shown from the results of Table 1in JP-A 195251/1999 wherein variation of the coating thickness is shownin relation to the outer diameter of the hollow cylinder of from 4 to 16mm, the unevenness of the thickness depends on the outer diameter of thehollow cylinder, and the degree of the coating unevenness increases withthe increase in the outer diameter of the hollow cylinder. To be morespecific, even when the resin is supplied to the interior of the hollowcylinder, the starting point of the coating will be deviated from thecenter of rotation, and the starting point of the coating will be theposition where the outer periphery of the hollow cylinder is located. Inconsideration of the relatively high viscosity of the resin, the outerdiameter of the hollow cylinder cannot be reduced to below 4 mm.Therefore, the thickness unevenness of the resin coating is difficult toreduce by the method described in JP-A 195251/1999.

In contrast to such conventional plug means, the plug means 300 shown inFIG. 6 comprises a disk member 301 and a support shaft 302 providedthereon. Therefore, handling of the plug means 300 in the production ofthe medium is easy, and in particular, removal of the plug means 300after spin coating is facilitated.

JP-A 195251/1999 discloses a plug means constituted from a cap and asupport member comprising a hollow cylinder or a plurality of rodsintegrally formed with the cap. The plug means shown in FIG. 6 hasvarious advantages over such plug means as described below.

In JP-A 195251/1999, the resin is once blocked at the wall or rods ofthe support member, and this may cause an uneven coating as describedabove. In contrast, in the plug means shown in FIG. 6, the spin coatingis accomplished by supplying the coating solution on the outer peripheryof the support shaft and the thickness evenness is unlikely to beinduced. In addition, in the case of the plug means shown in FIG. 6, thepart to which the resin attaches is the outer periphery of the supportshaft, and cleaning of the plug means is easy compared to JP-A195251/1999. Furthermore, the coating solution is supplied in JP-A195251/1999 to the interior of the hollow cylinder and the outerdiameter of the support member can not be reduced below certain level ifthe mobility of the coating solution with relatively high viscosity isto be ensured, and accordingly, the starting point of the coating isrelatively remote from the center of rotation. In contrast, the outerdiameter of the support shaft of the plug means shown in FIG. 6 can bereduced to a value significantly smaller than that of JP-A 195251/1999,and the thickness unevenness of the coating can be markedly reducedcompared to JP-A 195251/1999.

It should be noted that such advantages are realized not only by theplug means constituted as shown in FIG. 6, and as long as the plug meansis constituted from a disk member and a support shaft, the advantagesmaybe realized. The plug means 300 shown in FIG. 6 comprises afrustoconical disk member 301 and a columnar support shaft 302. Otherembodiments of the plug means which may be adopted include those havingthe structures as shown in FIGS. 14A to 14D.

The plug means shown in FIG. 14A comprises a frustoconical disk member301 having bored lower surface as in the case of FIG. 13 and an invertedfrustoconical support shaft 302. When such plug means is used, thestarting point for the coating of the coating solution can be broughtnearer to the center of the disk member 301 to further reduce thethickness unevenness of the coated layer. In addition, provision of suchsupport shaft 302 is not associated with decrease in its mechanicalstrength in contrast to the case wherein the diameter of the entiresupport shaft 302 is reduced. Furthermore, the plug means of suchconfiguration is advantageous in attaching/detaching and carrying of theplug means since gripping of the support shaft 302 with a chuck and thelike is facilitated to avoid dropping. It should be noted that not theentire support shaft 302 needs to be inverted frustoconical. To be morespecific, at least a part of the support shaft 302 should befrustoconical with its diameter reducing toward the disk member 301 andthe remaining portion of the shaft on the side of the disk member maynot necessarily be tapered as long as the diameter does not increasetoward the disk member.

The plug means shown in FIG. 14B is different from the one shown in FIG.14A in its cross-sectional configuration of the disk member 301. Inorder to facilitate consistent spreading of the coating solution on thedisk member 301, the disk member 301 may preferably have a thicknessgradually decreasing toward its outer periphery. In such case, the uppersurface where the coating solution spreads may have a rectilinear crosssection as shown in FIG. 14A or a curved cross section as shown in FIG.14B. In addition, the disk member 301 may have a vertical outerperiphery as shown in FIG. 14C, and in such as case, the thickness t ofthe disk member 301 at its periphery is preferably up to 0.4 mm sinceeven coating of the resin layer is difficult at an excessively largethickness t. The disk member 301 may also have an even thickness asshown in FIG. 14D.

The plug means shown in FIGS. 14A to 14D are those wherein lower surfaceof the disk member 301 has been bored to correspond to the formation ofthe second or the following resin layer.

In the plug means, the minimum diameter of the support shaft 302 nearthe disk member 301 is preferably less than 4 mm, and more preferably 2mm or less. When the diameter of the support shaft 302 near the diskmember 301 is too large, the starting point of the coating will beremote from the center of the disk member 301 to invite an increase inthe thickness unevenness in the radial direction of the resin layer.When the diameter of the support shaft 302 near the disk member 301 istoo small, the support shaft 302 will suffer from insufficientmechanical strength. Therefore, the minimum diameter is preferably atleast 0.5 mm, and more preferably at least 0.7 mm. The support shaft 302is not limited for its length, and an adequate length may be selected bytaking ease of supplying the coating solution to the outer periphery,ease of the gripping and the like into consideration. The length of thesupport shaft, however, is preferably in the range of 5 to 100 mm, andmore preferably 10 to 30 mm. An excessively short support shaft 302 isassociated with difficulty in supplying the coating solution to itsouter periphery as well as difficulty in the gripping. In contrast, anexcessively long support shaft 302 suffers from handling inconvenience.

The diameter of the disk member 301 is not limited as long as it islarger than the diameter of the center hole 101 of the disk substrateand smaller than the inner diameter of the annular information recordingarea of the disk substrate. The diameter of the disk member 301,however, is preferably at least 4 mm, and more preferably at least 8 mmlarger than the diameter of the center hole 101 in consideration of therisk of the coating solution 500 going underneath the disk member 301onto the periphery of the center hole 101 (inner periphery of the disksubstrate). The diameter of the disk member 301 is preferably at least 3mm, and more preferably at least 5 mm smaller than the inner diameter ofthe information recording area since the configuration of the resinlayer near the disk member 301 may be affected by the removal of thedisk member 301. Although the diameter of the disk member 301 employedmay vary according to the diameter of the center hole and the innerdiameter of the information recording area, the diameter of the diskmember 301 is typically in the range of 20 to 40 mm, and preferably 25to 38 mm when the optical disk has a diameter of about 60 to 130 mm.

The material used for constituting the plug means is not particularlylimited and the material may be a metal, a resin, a ceramics, or acomposite material containing two or more such materials. The diskmember 301 and the support shaft 302 may also comprise differentmaterials. In view of the mechanical strength, durability, anddimensional precision, the plug means preferably comprises a metal.Exemplary preferable metals include stainless steel alloy, aluminum, andaluminum alloy.

Surface tension of the surface of the plug means 300, and in particular,the entire surface of the disk member 301 is preferably lower than thatof the coating solution. When the surface of the plug means 300 is lesswettable to the coating solution, removal by washing of the coatingsolution that has attached to the surface of the plug means will befacilitated. The surface tension of the plug means can be controlled byadequately selecting the material used for the plug means. However, itis more preferable that the area where surface tension is to be reducedis subjected to a water- and oil-repellent treatment such as treatmentwith teflon.

Servo Layer

The servo layer is a reflective layer formed on the servo substrate 20,and the servo layer is formed with the projections and depressionscarrying the tracking servo information. The servo layer carriestracking servo information corresponding to the projections anddepressions. Grooves and/or pits are typically used for the projectionsand depressions.

The reflective layer constituting the servo layer is not particularlylimited for its constitution, and the reflective layer formed may besimilar to those formed in conventional optical information media. Thereflective layer may typically comprise a metal or semimetal such as Al,Au, Ag, Pt, Cu, Ni, Cr, Ti, or Si as a simple substance or as an alloycontaining at least one of such metals and semimetals. The reflectivelayer is typically formed to a thickness of 10 to 300 nm. A thicknessbelow such range is likely to result in an insufficient reflectivitywhile the thickness in excess of such range is not advantageous in costsince increase in the thickness does not result in significant increaseof the reflectivity. The reflective layer is preferably formed by vapordeposition such as sputtering and evaporation.

Data Layer

When the present invention is applied to an optical recording medium,the data layer includes at least a recording layer comprising arecording material. The optical recording medium to which the presentinvention is applied is not limited particular type, and applicablemedia include a rewritable medium or a write once medium employing aphase change recording material, a rewritable medium employing amagnetooptical recording material, a write once medium employing anorganic dye. However, use of a phase change recording material ispreferable in view of high light transmittance compared to otherrecording materials, and accordingly, capability of increasing thenumber of recording layers.

The composition of the phase change recording material is notparticularly limited, and the material is preferably the one containingat least Sb and Te. However, crystallization temperature of therecording layer containing Sb and Te as the only components is as low asabout 130° C. and the storage reliability is insufficient, andtherefore, the recording layer may preferably comprise elements otherthan Sb and Te. Such element is preferably element M (element M is atleast one element selected from In, Ag, Au, Bi, Se, Al, P, Ge, H, Si, C,V, W, Ta, Zn, Ti, Ce, Tb, Sn, Pb, Pd, and Y), and among these, thepreferred is Ge in view of the high effect in improving the storagereliability.

When the atomic ratio of the elements constituting the recording layeris represented by the formula (I):

Sb_(a)Te_(b)M_(c)  (I),

wherein a+b+c=1, a, b, and c are preferably such that:

0.2≦a≦0.85,

0.1≦b≦0.6, and

0≦c≦0.25, and more preferably,

0.01≦c≦0.25.

When the content of Sb is too low, crystallization speed will beinsufficient and the overwriting will be difficult. On the other hand,when the Sb content is too high, crystallization speed will beexcessively high and formation of amorphous record marks will bedifficult. When the content of M is too low, the effect of M additionwill be insufficient while addition of M in an excessive amount willresult in insufficient alteration of the reflectivity with the phasechange, and hence, in an insufficient degree of modulation. When Tecontent is too low, formation of record marks will be difficult due tothe difficulty in amorphization. On the other hand, when the Te contentis too high, crystallization speed will be insufficient and overwritingwill be difficult.

A phase change recording medium is generally used as a rewritablemedium. In the present invention, however, the phase change recordingmedium may be used as a write once medium. The “write once medium” usedherein designates a medium which is recordable but wherein erasure ofthe once recorded record mark is not ensured, and in the case of a writeonce medium, overwriting of the record marks recorded in the recordingtrack by erasing the record marks is not intended. Advantages associatedwith the use of a phase change recording medium for the write oncemedium are as described below.

In the case of a multi-layer recording medium, a plurality of recordinglayers are disposed one on another, and this structure is accompaniedwith an increased loss in the light quantity of the recording/readingbeam. Therefore, use of a thinnest possible recording layer is desired.Decrease in the thickness of the recording layer, however, invites anincrease in the cooling speed of the recording layer after the recordingbeam irradiation. Crystallization is less likely to take place at ahigher cooling speed, and use of a composition which easily undergocrystallization is required to ensure the erasability. In other words,considerable increase in the crystallization speed of the recordinglayer will be required. A recording layer of high crystallization speed,however, is associated with the problem of higher occurrence of the selferase as described below. In the recording, heat dissipates from thebeam spot of the recording beam in the lateral direction of therecording layer, and cooling of the record marks is inhibited by thisheat. When the recording layer has a high crystallization speed, therecord marks are partly recrystallized due to such cooling inhibition,and the size of the record mark formed will be reduced. To be morespecific, the phenomenon often encountered is erasure of the leadingedge of the record mark (the part first irradiated with the beam spot)or the trailing edge of the record mark. Such phenomenon is referred inthe present invention as the “self erase”. The self erase is associatedwith decrease in the C/N or increase in the jitter.

As described above, when the thickness of the recording layer isreduced, it will be difficult to simultaneously ensure sufficienterasability and suppress the self erase. In contrast, when a mediumhaving a phase change recording layer is used as a write once medium,there will be no need to erase the record marks, and hence, to considerthe crystallization speed of the recording layer. Accordingly, noproblem will be induced even if the crystallization speed of therecording layer were reduced to the level where no substantial influenceon the self erase is induced. In addition, when overwriting isconducted, increase in the crystallization speed of the recording layeris required with the increase in the linear velocity of the medium inthe recording, and this also invites increased likeliness of self erase.However, if the recording is conducted only once with no overwritingoperation, it will be possible to conduct the recording at a high linearvelocity, for example, at a linear velocity of about 10 m/s in arecording layer having a relatively slow crystallization speed withreduced likeliness of self erase, and a high data transfer rate iseasily realized.

As described above, the medium of the present invention has a pluralityof recording layers disposed one on another and loss of the lightquantity of the recording/reading beam is accordingly increased.Therefore, use of a thinnest possible recording layer is preferable withthe function of the recording layer maintained. However, an excessivelythin recording layer can no longer function as a recording layer, andthe recording layer preferably has a thickness of 2 to 50 nm, and morepreferably, 4 to 20 nm.

When a phase change recording layer is employed, the data layer maypreferably have the structure as shown for DL-1 in FIG. 3. This datalayer has a structure wherein the recording layer 4 is sandwichedbetween the first dielectric layer 31 and the second dielectric layer32. When such structure is adopted, the recording layer and thedielectric layers are preferably formed by sputtering. The dielectricmaterial used in the dielectric layers may be a compound containing atleast one metal component selected from Si, Ge, Zn, Al, and rare earthmetals, and the compound is preferably an oxide, a nitride, a sulfide,or a fluoride. A mixture containing two or more such compounds may bealso used. Each dielectric layer may preferably have a thickness of 10to 500 nm.

In the structure shown in FIG. 1, the data layer DL-2 which is thefurthest data layer from the side of the recording/reading beamincidence generally comprises a laminate of the reflective layer, thedielectric layer, the recording layer, and the dielectric layer disposedin this order from the upper side of the drawing. On the other hand, noreflective layer is formed in the case of the data layer DL-1 of FIG. 1to facilitate the transmittance of the recording/reading beamtherethrough. However, the data layer DL-1 may optionally include areflective layer which is translucent to the recording/reading beam, andin such a case, the data layer DL-1 will have a structure similar tothat of the data layer DL-2.

In the present invention, use of the recording layer with a reducedthickness is preferable in order to reduce the loss in the lightquantity of the recording/reading beam. Decrease in the thickness of thephase change recording layer, however, is associated with a decrease inthe degree of modulation, namely, with a decrease in the difference inreflectivity between the amorphous record mark and the crystallineregion. In order to increase the degree of modulation, the dielectriclayer is preferably formed as a laminate of two or more layers eachhaving different refractive index. Such multi-layer structure alsoresults in an increased flexibility of optical design, and increase inthe light transmittance of the entire data layer can be realized. Anexemplary dielectric layer of multi-layer structure is a laminate of atleast one layer selected from magnesium fluoride layer, manganesefluoride layer, germanium nitride oxide layer, and silicon oxide layerwith a ZnS—SiO₂ layer.

When a plurality of recording layers are formed, intensity of therecording beam reaching the particular recording layer reduces with theincrease in the distance of the recording layer from the surface of therecording beam incidence into the medium. Therefore, recordingsensitivity of the recording layer is preferably adjusted correspondingto the intensity of the recording beam reaching to the particularrecording layer. In the case of recording materials wherein heat moderecording is conducted as in the case of phase change recordingmaterials, increase in the thickness of the recording layer results inan increase in heat storage, and hence, in an increase in the recordingsensitivity. In view of such situation, the thickness of the recordinglayer remote from the surface of the recording beam incidence may beincreased as required compared to the recording layer near the surfaceof the recording beam incidence. However, adjacent two recording layersmay have an identical thickness. In addition, the recording/reading beamused in the recording layer remote from the surface of the recordingbeam incidence is the recording/reading beam which has passed throughother recording layers, and for the purpose of leveling the readingproperties of the recording layers, a recording layer near the surfaceof the recording beam incidence may preferably have a higher lighttransmittance. In consideration of such light transmittance, it is alsopreferable that the recording layer remote from the surface of therecording beam incidence has an increased thickness.

It should be noted that the adjustment of the recording sensitivity andthe transmittance can also be accomplished through control of thecomposition of the recording layer. In such case, all recording layersmay be formed to an identical thickness, or alternatively, control ofthe composition can be combined with the control of the thickness.

The present invention is also applicable to a read only medium. The datalayers of such medium may comprise either a layer formed with pitscarrying the recorded information or a layer of a write once mediumcarrying the preliminarily recorded data. In the former case, the pitsare generally formed in the transparent layer or the filter layer, and atranslucent reflective layer is formed on the surface of the layerformed with the pits. The reflective layer will then serve as the datalayer. Examples of such translucent reflective layers are an extremelythin metal layer and a silicon layer. In such read only medium,reflectivity of the data layer may be controlled for the leveling of theread-out signal. To be more specific, the reflectivity may be controlledsuch that the data layer with the smaller quantity of light reachedexhibits higher reflectivity. When the reflectivity is controlled asdescribed above, the data layer near the surface of the beam incidencewill exhibit higher light transmittance and marked attenuation in thequantity of the light reaching the data layer remote from the surface ofthe beam incidence will be avoided.

In the present invention, number of the data layers included in themedium is not limited as long as two or more data layers are included.An excessive number of data layers, however, results in unduly increasedthickness of the medium and the effect of the thickness inconsistency ofthe transparent layer formed by spin coating will also surpass theacceptable level. Accordingly, the number of the data layers ispreferably up to 10, and more preferably up to 6.

When a plurality of information-storing layers are disposed one onanother, quantity of the light reflected from the information-storinglayer will be reduced. However, it has been found in the investigationof the inventors of the present invention that sufficient C/N at thedata layer and sufficient servo signal at the servo layer are attainedwhen the maximum reflectivity of the information-storing layer is 5% orless. However, sufficient C/N and servo signal intensity will not beensured when the reflectivity is excessively low, and theinformation-storing layer may preferably have a maximum reflectivity ofat least 0.1%.

Substrate 2 and Servo Substrate 20

The substrate 2 preferably comprises a material which is substantiallytransparent to the recording/reading beam such as a resin or glass sincethe recording/reading beam is irradiated through the substrate 2. Amongsuch materials, use of a resin is preferable in view of the handlingconvenience and the low price, and exemplary resins include acrylicresins, polycarbonates, epoxy resins, and polyolefins. However, when therecording/reading beam used has a wavelength as short as about 450 nm orbelow, a polycarbonate substrate will exhibit an excessively highabsorption of the recording/reading beam, and use of a material such asan amorphous polyolefin exhibiting lower optical absorption to a shortwavelength beam is preferable.

The substrate 2 is not limited for its shape and dimension. Thesubstrate 2, however, is typically a disk having a thickness of at least5 μm and preferably about 30 μm to 3 mm and a diameter of about 50 to360 mm.

The servo substrate 20 shown in FIG. 3 may comprise a resin or a glassas in the case of the substrate 2. Use of a resin, however, ispreferable in view of the ease of forming the servo information-carryingprojections and depressions by injection molding. It should be notedthat the servo substrate 20 is not necessary transparent. The servosubstrate 20 is also not limited for its thickness, and an adequatethickness may be selected, for example, from the range described for thesubstrate 2. However, when the substrate 2 has a relatively lowrigidity, the rigidity of the entire medium is preferably ensured byincreasing the thickness of the servo substrate 20 to a considerabledegree.

EXAMPLES Example 1

A sample of optical recording disk having the structure as shown in FIG.3 was produced by the procedure as described below.

4 transparent layers TL-1 to TL-4 and 4 data layers DL-1 to DL-4 werealternately disposed on one surface of a substrate 2 comprising a glassdisk which had been toughened on both surfaces (thickness, 1.2 mm;diameter 120 mm).

The transparent layers were formed by spin coating a UV-curable resin(SK-5110 manufactured by Sony Chemicals Corporation) at a rotation speedof 1500 rpm for 2 seconds and UV curing the coated resin. Thetransparent layer after the curing had a thickness of 15 μm. It shouldbe noted that this thickness is a value measured at the radially centralposition of the area where recorded information is carried (the area ata radial distance of 20 mm to 58 mm from the center of the disk).

The recording layer 4 included in each data layer had a composition(atomic ratio) of:

Sb_(22.1)Te_(56.0) _(Ge) _(21.9)

The recording layers 4 were formed to a thickness of 5 nm, 5 nm, 7 nm,and 13 nm, respectively, from the side of the data beam incidence. Therecording layer 4 was formed by magnetron sputtering system and thethickness was adjusted by controlling the power, pressure, and time ofthe sputtering.

The first dielectric layer 31 and the second dielectric layer 32included in each data layer was adjusted to the range of 75 to 271 nm tothereby ensure absorption of the recording layer and simultaneouslyincrease the light transmittance of the entire data layer. Thesedielectric layers were formed by magnetron sputtering system and thecomposition of the layers was ZnS (80 mole %)—SiO₂ ₍20 mole %).

In the meanwhile, a servo substrate 20 comprising a disk having athickness of 1.2 mm and a diameter of 120 mm was prepared by injectionmolding a polycarbonate. This disk had a groove having a width of 0.76μm and a depth of 183 nm formed therewith. On the surface of the servosubstrate 20 where the groove had been formed, a gold layer wasdeposited as a servo layer SL to a thickness of 100 nm by sputtering. Onthe surface of this reflective layer was formed a filter layer FL, andthe filter layer FL was formed by spin coating a mixture (dye content, 3mass %) of a phthalocyanine dye (Blue-N manufactured by Nippon KayakuCo., Ltd.) and a UV-curable resin (SK-5110 manufactured by SonyChemicals Corporation) at a rotation speed of 2500 rpm for 5 seconds andUV curing the layer. The filter layer FL after curing had a thickness of11 μm. The filter layer FL exhibited an absorption of 95% at awavelength of 660 nm, and 8% at a wavelength of 780 nm. It should benoted that the absorption is a value evaluated by forming the filterlayer alone on a transparent plate, and conducting the evaluation forthis sample.

Next, UV-curable resin (DVD-003 manufactured by Nippon Kayaku Co., Ltd.)was dripped on the top surface of the laminate including the substrate 2(surface of the uppermost data layer DL-4), and the laminate includingthe servo substrate 20 was aligned on the laminate including thesubstrate 2. The laminates were rotated at 5000 rpm for 2 seconds, andthe UV-curable resin was cured by UV irradiation through the substrate 2to thereby adhere the laminate including the substrate 2 and thelaminate including the servo substrate 20 by an intervening transparentlayer TL-5 of 35 μm thick. A sample of the optical recording disk havingthe structure shown in FIG. 3 was thereby produced.

The recording layers of this sample were initialized (crystallized) witha bulk eraser. The data layers and the servo layer were evaluated fortheir maximum reflectivity at a wavelength of 660 nm.

DL-1: 1.1%,

DL-2: 0.7%,

DL-3: 0.9%,

DL-4: 0.5%,

SL: 0.05%.

Bit Contrast

The recording layers of this sample were initialized (crystallized) witha bulk eraser, and the sample was recorded by irradiating the samplewith a recording data beam having a wavelength of 660 nm and a pulsewidth of 50 ns through the substrate 2 with the sample in stationarystate, and the bit contrast of each layer was measure by irradiating thereading data beam having the same wavelength. An optical pickup having aconfocal optical system was used for the irradiation of the data beamand the detection of the reflected beam. The objective lens of theoptical pickup had a numerical aperture of 0.52. The results are shownin Table 1. The bit contrast shown in Table 1 is

(R ₀ −R ₁)/R ₀

wherein R₀ is the reflectivity before recording and R₁ is a reflectivityafter recording. The minimum power P_(min) shown in Table 1 is theminimum power of the recording data beam at which the contrast wasfound.

TABLE 1 Pmin Bit Contrast Data layer (mW) (%) DL-1 14 36 DL-2 17 27 DL-317 23 DL-4 15 47

The results shown in Table 1 reveal that the bit contrast produced wassufficient in all of the four data layers. It was also revealed that thedifference in the recording sensitivity between the data layers wassmall.

C/N (Carrier to Noise Ratio)

The sample was rotated, and single signal comprising pulses of the samelength continuing at a constant interval was recorded in the data layersof the sample, and the recorded signal was read to measure the C/N. Therecorded pulses were at a duty ratio of 50%. A data beam having awavelength of 660 nm was used for the recording and the reading. In therecording and the reading, tracking servo was also conducted by readingthe servo layer SL with a servo beam having a wavelength of 780 nm.

The results are shown in Table 2. The recording density shown in Table 2is the value determined by converting the mark length of the singlesignal as described above to the bit linear density of the 1-7modulation signal containing the signal of the same mark length as itsminimum signal. In the measurement, the disk was rotated at a rotationspeed of 2000 rpm (CAV), and the recording density was changed bychanging the frequency of the single signal as described above. Itshould be noted that the recording track measured was at a radialposition measured from the sample center of 42.5 mm, and the linearvelocity was about 8.9 m/s.

TABLE 2 C/N at 53.3 kBPI C/N at 80.0 kBPI Data layer (dB) (dB) DL-1 45.742.6 DL-2 42.5 39.2 DL-3 50.8 44.0 DL-4 45.8 43.1

The results shown in Table 2 reveal that the C/N was sufficiently highin the high density recording at 80 kBPI.

Bit Error Rate

The sample was recorded with a random signal of 1-7 modulation (marklength, 2T to 8T), and the signal was read to measure the bit error rate(BER). The results are shown in Table 3.

TABLE 3 Data layer BER at 84.0 kBPI DL-1 2.0 × 10⁻⁵ DL-2 3.5 × 10⁻⁵ DL-31.0 × 10⁻⁷ DL-4 1.0 × 10⁻⁷

The results shown in Table 3 reveal that the bit error rate wassufficiently low in the high density recording at 84 kBPI.

Example 2

Samples of optical recording disk were produced by repeating theprocedure of Example 1 except that the filter layer FL was formed by theprocedure as described below.

The filter layer FL of this sample was formed by spin coating a mixture(dye content, 3 mass %) of a yellow dye (Yellow-2G manufactured byNippon Kayaku Co., Ltd.) and a UV-curable resin at a rotation speed of2500 rpm for 5 seconds and UV curing the resin. The filter layer FLafter curing had a thickness of 10 μm. The filter layer FL exhibited anabsorption of 95% at a wavelength of 405 nm, and 7% at a wavelength of650 nm. It should be noted that the absorption was measured by repeatingthe procedure of Example 1.

The sample was evaluated for its recording/reading properties byrepeating the procedure of Example 1 except for the wavelength of thedata beam and the servo beam which were 405 nm and 650 nm, respectively.The properties were sufficient as in the case of Example 1.

Example 3

Samples of optical recording disk were produced by repeating theprocedure of Example 1 except that the filter layer FL was formed by theprocedure as described below.

The filter layer FL of this sample was formed by spin coating aUV-curable resin admixed with 3 mass % of Irgacure 819 (aphotoinitiator, manufactured by Ciba Specialty Chemicals Corporation) ata rotation speed of 2500 rpm for 5 seconds and UV curing the resin. Thefilter layer FL after curing had a thickness of 10 μm. The filter layerFL exhibited an absorption of 93% at a wavelength of 405 nm, and 5% at awavelength of 650 nm. It should be noted that the absorption wasmeasured by repeating the procedure of Example 1.

The sample was evaluated for its recording/reading properties byrepeating the procedure of Example 1 except for the wavelength of thedata beam and the servo beam which were 405 nm and 650 nm, respectively.The properties were sufficient as in the case of Example 1.

Example 4

Sample of the Present Invention

A sample of the optical recording disk having the structure as shown inFIG. 1 was produced by the procedure as described below.

A polycarbonate disk having a diameter of 120 mm and a thickness of 0.6mm was produced by injection molding with grooves having a width of 0.2μm and a depth of 0.03 μm formed on one surface of the disk at a pitchof 0.74 μm. This polycarbonate disk corresponds to the substrate 2 inFIG. 1. On the surface of this substrate 2 formed with the grooves wereformed a first dielectric layer, a phase-change recording layer, and asecond dielectric layer in this order by sputtering. The firstdielectric layer had the composition of ZnS (80 mole %)—SiO₂ ₍20 mole%), and this layer was deposited to a thickness of 140 nm. The recordinglayer had the composition:

Ag₅Ge₂In₂Sb₆₇Te₂₄ (atomic ratio)

and this layer was formed to a thickness of 8 nm. The second dielectriclayer had the composition of ZnS (80 mole %)—SiO₂ (20 mole %), and thislayer was deposited to a thickness of 130 nm. The recording layer wasthen initialized (crystallized) with a bulk eraser. This laminate of thesubstrate 2 and the data layer DL-1 is referred to as “the first disk”in this example.

Next, a polycarbonate disk having a diameter of 120 mm and a thicknessof 0.6 mm was produced by injection molding with grooves having a widthof 0.5 μm and a depth of 0.04 μm formed on one surface at a pitch of 1.6μm. This polycarbonate disk corresponds to the transparent layer TL inFIG. 1. On the surface of this transparent layer TL formed with thegrooves were formed a reflective layer, a first dielectric layer, aphase-change recording layer, and a second dielectric layer in thisorder by sputtering to thereby constitute the data layer DL-2. Thereflective layer was formed from aluminum and this layer was depositedto a thickness of 100 nm. The first dielectric layer had the compositionof ZnS (80 mole %)—SiO₂ (20 mole %), and this layer was deposited to athickness of 70 nm. The recording layer had the same composition as therecording layer of the first disk, and this layer was formed to athickness of 12 nm. The second dielectric layer had the composition ofZnS (80 mole %)—SiO₂ ₍20 mole %), and this layer was deposited to athickness of 140 nm. The recording layer was then initialized(crystallized) with a bulk eraser. This laminate of the transparentlayer TL and the data layer DL-2 is referred to as “the second disk” inthis example.

Next, on the data layer DL-2 of the second disk was spin coated amixture (dye content, 3 mass %) of a phthalocyanine dye (Blue-Nmanufactured by Nippon Kayaku Co., Ltd.) and a UV-curable resin (SK-5110manufactured by Sony Chemicals Corporation) at a rotation speed of 2000rpm for 2 seconds. The first disk was then placed on the second disk sothat the data layer DL-1 is in contact with the spin-coated layer, andthe spin coated layer was UV-cured. A sample wherein the first disk andthe second disk are adhered by the intervening filter layer FL wasthereby obtained.

The cured filter layer FL was measured with a laser interferometer, andthe thickness was 30 μm. The filter layer FL exhibited an absorption of96% at a wavelength of 660 nm, and 15% at a wavelength of 780 nm. Itshould be noted that the absorption is a value evaluated by forming thefilter layer alone on a transparent plate, and evaluating the sample.

The data layer DL-1 and the data layer DL-2 of this sample wereirradiated with laser beam through the substrate 2 to measure thereflectivity. The reflectivity at the wavelength of 660 nm was:

DL-1: 10%,

DL-2: 0.1%.

The sample was also evaluated for recording/reading properties of thedata layer DL-1 by an optical disk evaluator according to thespecification (DVD-RW ver. 1.0). The clock jitter was then measured tobe 7% indicating the favorable properties of the sample.

Comparative Sample

A comparative sample was prepared as in the case of the sample of theinvention except that a transparent layer of the UV-curable resin freefrom the dye was formed instead of the filter layer. The transparentlayer of this comparative layer was the one formed by spin coating aUV-curable resin (SK-5110 manufactured by Sony Chemicals Corporation) ata rotation speed of 1200 rpm for 1 second and curing the coated layer.The layer had a thickness of 30 μm, which is the same as the filterlayer FL. No absorption was observed for this transparent layer at thewavelength of 660 nm and 780 nm.

The data layer DL-1 and the data layer DL-2 of this comparative samplewere irradiated with laser beam through the substrate 2 to measure thereflectivity. The reflectivity at the wavelength of 660 nm was:

DL-1: 10%,

DL-2: 9%.

The sample was also evaluated for recording/reading properties of thedata layer DL-1 by an optical disk evaluator according to thespecification (DVD-RW ver. 1.0). The clock jitter was then measured tobe 13%, and some noise was observed. This noise was estimated to be dueto the leakage of the beam reflected from the data layer DL-2.

Merits of the Invention

In the present invention, two or more recording/reading beams eachhaving different wavelength are used for the multi-layer recordingmedium having a plurality of information-storing layers, and a filterlayer exhibiting selective absorption for the recording/reading beams isformed between the adjacent information-storing layers. As aconsequence, signal interference between the adjacentinformation-storing layers sandwiching the filter layer is reduced.

Japanese Patent Application Nos. 174542/2000 and 233782/2000 areincorporated herein by reference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in the light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

What is claimed is:
 1. An optical information medium comprising: atleast two information-storing layers each storing recorded informationand/or servo information, wherein at least one of theinformation-storing layers is recorded or read by a recording beam or areading beam which has passed through other information-storinglayer(s); and a filter layer between the adjacent information-storinglayers, wherein in the spectral absorption characteristics in awavelength range of 300 to 1000 nm of the filter layer, a firstwavelength range exhibiting a first absorption of 80% or higher and asecond wavelength range exhibiting a second absorption of 20% or lowerare present.
 2. The optical information mediun according to claim 1,wherein the filter layer is a layer containing a UV-curable compositionand a photopolymerization initiator.
 3. The optical information mediumaccording to claim 1, wherein the filter layer is a layer containing adye.
 4. An optical information medium comprising: at least twoinformation-storing layers each storing recorded information and/orservo information, wherein at least one of the information-storinglayers is recorded or read by a recording beam or a reading beam whichhas passed through other information-storing layer(s); and a filterlayer between the adjacent information-storing layers, wherein theoptical information medium is used in a system wherein two or morerecording/reading beams each having different wavelength are used,wherein the filter layer exhibits a relatively high absorption for thefirst reading/recording beam used for its closest information-storinglayer on a side of a light incidence and a relatively low absorption fora second reading/recording beam used for its closest information-storinglayer on a side of a light exit.
 5. The optical information mediumaccording to claim 4, wherein the filter layer exhibits an absorptionfor the reading/recording beam used for its closest information-storinglayer on the side of the light incidence of 80% or more and anabsorption for the reading/recording beam used for its closestinformation-storing layer on the side of the light exit of 20% or lower.6. The optical information medium according to claim 4, wherein thefilter layer is a layer containing a UV-curable composition and aphotopolymerization initiator.
 7. The optical information mediumaccording to claim 4, wherein the filter layer is a layer containing adye.